The research was an attempt to establish if, by studying particular physical characteristics normally associated with 'race', if it was possible to determine the genetic background of a person. The results appear in today's issue of the journal, Proceedings of the National Academy of Sciences.

Dr Sergio Pena and colleagues used a set of genetic markers to test how closely physical 'race-determining' traits correlated with ancestry in Brazil. They chose Brazil as their target population because the country has a long history of inter-ethnic mixing between European, African and Native American lineages.

The group isolated three samples of populations considered representative of the major founding groups of Brazil: Portugal, Africa and Native Americans.

"We wanted to ascertain to what degree the colour of a Brazilian individual was predictive of the degree of genomic African ancestry," Pena wrote.

The first test group consisted of 173 people living in a rural community who were classified as 'white', 'black' or 'intermediate' based on arm skin pigmentation, hair colour and nose and lip shape. The second consisted of 200 men living in major metropolitan areas of Brazil who classed themselves as 'white', or Caucasian.

The researchers used a genetic marker that distinguishes between people from Portugal and those from Africa. The number of times the African marker appeared was reflected in an African Ancestry Index (AAI) for each person. These AAI values can reliably distinguish between Europeans and Africans.

People who looked 'black' might have expected to score a higher AAI. But in the populations tested, there was little difference in AAI values between groups - whether they were initially classed as black, white or intermediate.

The self-nominated 'white' urban men tested in the second group also showed AAI values halfway between Europeans and Africans.

"It is interesting to note that the group of individuals classified as blacks had a very high proportion of non-African ancestry (48 per cent)," the authors said. "The intermediate group, with 45 per cent African ancestry, was closer to the black group than to the white one."

The concept of race - classifying humans into different groups based on physical characteristics that indicate a lineage or 'breed' - has been around for centuries, but has no basis in science, the scientists said. The new study further supports this view, and for the first time quantifies it.

It was an old and now discredited concept in science that was based on flawed evidence and personal prejudice, he said. "Race is many things to many people, and amongst other things it is a former scientific concept, that on the whole is not found to be much use in science these days," he told ABC Science Online.

Despite this, the way people look strongly affects how others respond to them and what 'race' they are considered part of, the Brazilian and Portuguese scientists said. Their study shows that, in Brazil at least, looks are a highly inaccurate way to determine a person's genetic ancestry.

Attenborough said that the study differs from previous work because - rather than looking at a whole population - it analyses the data at the level of individual people. It is much easier to find patterns in genetic traits when looking at whole populations.

"There are differences between groups of people, but it is much harder to find clear cut distinctions," said Attenborough. "Nobody disputes that - biologically - individual human beings differ, and if you put together groups with a similar origin, then there will be average differences between groups with one origin and groups with another origin."